1. Field of the Invention
This invention relates to a noise reducing circuit for reducing noises of a receiver such as an FM receiver in accordance with fuzzy control of a control parameter of any of a frequency characteristic control circuit, a separation control circuit and a muting control circuit or of each of any combination of such circuits.
2. Description of the Prior Art
Generally, a receiving condition of a receiver is influenced significantly by receiving environment. Particularly in the case of an FM receiver, a receiving condition is varied greatly by multipath disturbance caused by geography of a circumferential place or by a difference in arrival time of reflected waves of linearly advancing waves due to the linearly advancing property of FM broadcasting waves.
In this manner, the reception electric field strength of a receiver is liable to be influenced by circumferential environment. The degree of such variation is higher with a receiver which is carried on a moving body such as an automobile because the surrounding environment is varied rapidly in accordance with movement of the moving body.
When a reception electric field strength and/or a mutipath condition are varied by a change of circumferential environment in this manner, the S/N ratio or the output of the receiver is varied. Thus, various inventions have been made for a car-carried receiver so that, even if the reception electric field strength and/or the multipath condition are varied by a variation of the surrounding environment, the S/N ratio or the output may not be varied. Particularly, since an FM stereo receiver presents a greater deterioration in sound quality caused by a deterioration of the S/N ratio upon dropping of a reception electric field strength value than an FM monaural receiver, special inventions have been made for FM stereo receivers.
FIG. 10 shows in block diagram an exemplary one of conventional car-carried FM stereo receivers. Referring to FIG. 10, in the FM stereo receiver shown, an FM stereo signal of a high frequency inputted from an antenna 21 is amplified and converted into an intermediate frequency (IF) signal by a front end section 22.
The IF signal is amplified by an IF amplifier 23 having a limiter function and then converted into an FM composite signal by an FM detecting circuit 24. The FM composite signal is amplified by an amplifier 25 and then passes through a muting control circuit 26, whereafter it is separated into left (L) and right (R) channel signals by a multiplex (MPX) demodulating circuit 27.
The left and right channel signals are supplied to a separation control circuit 28 for controlling a degree of separation of left and right channel signals and then pass through a frequency characteristic control circuit 29 for attenuating a high frequency level of left and right channel signals. Then, the left and right channel signals are outputted separately from the frequency characteristic control circuit 29, that is, from the FM stereo receiver.
Meanwhile, a level detecting circuit 31 of AM detection construction such as a so-called S meter detects a signal level of the IF amplifier 23 as a signal level which increases in proportion to a reception electric field strength value. The signal level thus detected is supplied to a pair of level setting circuits 32 and 33.
An output signal of the level setting circuit 32 is inputted to a control signal generating circuit 34, so that a control signal A corresponding to the TF signal level is produced from the control signal generating circuit 34. The control signal A causes the muting control circuit 26 to start its variable attenuating operation when the reception input level, that is, the reception electric field strength value, drops below a certain value, but when the reception input level further drops, the control signal A causes the attenuating amount of the muting controlling circuit 26 to be varied continuously in response to the reception input level.
A frequency displacement detecting circuit 35 instructs, when the input signal level is further dropped by a frequency displacement or the like and the output of the amplifying circuit 25 is dropped accordingly, the control signal generating circuit 34 to reduce the attenuating amount of the muting control circuit 26 infinitely to put the FM stereo receiver into a muted condition.
An output of the other level setting circuit 33 is inputted to a time constant circuit 36 at which a control signal B having suitably controlled rising and falling edge characteristics is produced. The frequency response of the frequency characteristic control circuit 29 is controlled in response to the control signal B.
Meanwhile, the control signal B is converted into a suitable level by a level controlling circuit 37 to make a separation degree adjusting signal C for the separation controlling circuit 28.
Referring now to FIG. 11, there are shown operation characteristics of of the FM stereo receiver. When the reception electric field strength value drops until the level detection value of the level detecting circuit 31 becomes lower than a first value E.sub.1 (for example. 45 dB/.mu.V or so) shown in FIG. 11, the separation control circuit 28 first starts its operation in response to a separation degree adjusting signal C generated by the level controlling circuit 37. Consequently, as the reception electric field strength value drops, the degree of stereo separation drops while such drop of the S/N ratio is reduced. In case the separation control circuit 37 starts its operation when the reception electric field strength value is 45 dB/.mu.V or so, a monaural operation will be entered from a stereo operation at 25 dB/.mu.V or so.
When the reception electric field strength value drops until the level detection value of the level detecting circuit 31 becomes lower than a second value E.sub.2 (for example, 35 dB/.mu.V or so) shown, the frequency characteristic control circuit 29 operates in response to a control signal B generated by the time constant circuit 36, and subsequently, the attenuation amount in a high frequency region increases as the reception electric field strength value drops. Consequently, while the high frequency characteristic of left and right channel signals drops as the reception electric field strength value drops, the drop of the S/N ratio is reduced. In case the frequency characteristic control circuit 29 operates when the reception electric field value is 35 dB/.mu.V or so, frequency characteristic control for a high frequency region up to 15 dB/.mu.V or so is performed.
When the reception electric field strength value further drops until the level detection value of the level detecting circuit 31 becomes lower than a third value E.sub.3 (for example, 25 dB/.mu.V or so) shown, the muting control circuit 26 operates in response to a control signal A generated by the control signal generating circuit 34, and subsequently, the attenuation amount of left and right channel signals is continuously increased as the reception electric field strength value drops. Consequently, while the output level of left and right channel signals drops as the reception electric field strength value drops, increase of the noise level is reduced.
When the reception electric field strength value further drops until the level detection value of the level detecting circuit 31 becomes lower than a fourth value E.sub.4 (for example, 15 dB/.mu.V or so) shown, the control signal generating circuit 34 operates in response to a control signal generated by the frequency displacement detecting circuit 35 so that the attenuation amount of the muting control circuit 26 becomes infinite to put the FM stereo receiver into a muted condition.
Thus, when the receiving input level, that is, the detection value of the reception electric field strength value, becomes lower than the first value, separation of left and right channel signals is varied continuously, and when the detection value becomes lower than the first value E.sub.1 and further lower than the second value E.sub.2 within a separation control range, the frequency characteristic with regard to a reproduction signal is varied continuously, and further, when the detection value becomes lower than the second value E.sub.2 and further lower than the third value E.sub.3 around a monaural operation mode by separation control, the reproduction signal revel is varied continuously.
Since the characteristics of the separation control circuit 28, frequency characteristic control circuit 29 and muting control circuit 26 are varied continuously from operation starting points of the individual circuits as the reception input level, that is, the reception electric field strength value, drops in this manner, a substantially constant S/N ratio can be assured and the noise level can be reduced independently of a magnitude of the reception electric field strength value as indicated by a broken line curve in FIG. 11.
Meanwhile, since a good S/N ratio and noise level are assured irrespective of a magnitude of the reception electric field strength value and the characteristics of the individual circuits are controlled continuously, even if the reception electric field strength value varies, an output signal of the FM stereo receiver will not vary suddenly, and it is possible to eliminate an unfamiliar feeling to a variation of the output signal of the FM stereo receiver.
As can be understood from the foregoing description of the controlling system of characteristics of the individual circuits in the FM stereo receiver, separation control, frequency characteristic control and muting control in a conventional FM receiver are of the nature that part of a signal received is omitted or processed in principle. Consequently, improvement in noise reduction effect and improvement in reduction effect of an unfamiliar feeling and an erroneous operation thereof have a mutually antinomic relationship. This similarly applies to a pulse noise suppressing system for removing external pulse noises.
Due to such circumstances, in a conventional noise reducing system for an FM receiver, control characteristics of individual control circuits are selectively determined while setting the point of compromise between a noise reducing effect and a reducing effect of an unfamiliar feeling and an erroneous operation uniformly.
By the way, for example, the multipath amount may be great or small with an equal detection value of the reception electric field strength. In such instance, it is possible to realize further good controls by setting control values of the individual control circuits individually in accordance with a relationship in magnitude of multipath amounts. This similarly applies to any other amount which defines a receiving condition.
In particular, it is possible to reconcile improvement in pulse noise reducing effect and a reducing effect of an unfamiliar feeling and an erroneous operation in high dimensions with each other in accordance with a receiving input condition by individually controlling control object amounts of individual control circuits in response to magnitudes of levels of individual factors which define such receiving input condition as described above.
However, in a conventional receiver, control parameters such as controlling time constants, controlling operation starting voltages, controlling amounts and so forth of the individual control circuits are set uniformly without taking contents of individual factors defining a receiving input condition into consideration. Consequently, it is not possible to variably set controlling conditions of the individual control circuits individually in accordance with conditions of receiving circumstances, and there is a disadvantage that it is difficult to realize it to make improvement in pulse noise reducing effect and a reducing effect in unfamiliar feeling and erroneous operation in accordance with contents of individual factors defining a receiving input condition.